The present invention relates to lasers, and more particularly, to a tunable rare-earth doped laser. The doped laser is implemented in an optical transmission medium such as an optical fiber or silica waveguide. The rare-earth material can comprise, for example, erbium or neodymium.
Applications for optical fiber technology are expanding at a rapid pace. Telecommunication, sensors, medical and video transmission can all take advantage of optical technology, particularly where virtually unlimited bandwidth and low attenuation are beneficial. Cable television systems are one example where optical fiber technology is providing efficient and economical alternatives to prior coaxial cable distribution schemes.
Many applications for fiber optic technology would be more cost effective if higher power lasers operating in the 1.3 or 1.5 micron (".mu.m") region were available. It has been demonstrated that a Er.sup.3+ fiber laser can produce high levels of power. See, e.g., M. S. O'Sullivan, et al, "High Power Narrow Linewidth Erbium-Doped Fiber Laser", CLEO 1989, Tup 3, pp. 134-136.
The wavelength emitted by a laser, including a rare-earth doped laser, is not absolutely stable. Various factors, such as temperature fluctuations, can influence the actual oscillation wavelength of a laser. The gradual drift of laser wavelength can change how much light is emitted at the wavelengths corresponding to different longitudinal modes. In some cases, this can lead to "mode hopping", where the laser shifts quickly from one dominant mode to another, at a slightly different wavelength.
Laser cavity optics normally reflect light over a bandwidth that is wide compared to the laser's gain bandwidth. Various devices have been used, such as prisms and gratings, to change the wavelength that can oscillate in the laser cavity, thereby tuning the laser. As long as the selected wavelength remains in the laser's gain bandwidth, the laser can operate. Because all laser media have gain only over limited ranges of wavelengths, their tuning ranges are limited. Since gain influences the power level available from the laser, the output power will change as the laser's wavelength is changed. It is therefore necessary to provide means for tuning a laser, such as a rare-earth doped laser.
It would be advantageous to provide a tunable fiber laser wherein the tuning can be accomplished using an electrical control circuit. It would be further advantageous to provide an electrical tuning system for a rare-earth doped fiber laser of the type that can produce a clean optical carrier for signal transmission.
The present invention provides a laser tuning system having the aforementioned advantages.